Recent clinical trials have substantiated hydroxyurea (HU) to be a potent effector of hemoglobin F (HbF) production in patients with sickle cell disease. This augmentation in HbF levels in patients is associated with a decrease in relative beta synthesis and thus an inhibitory effect of HbF. In order to gain insight into the possible genetic mechanisms underlying these effects, we used cation exchange (CE) and reverse-phase high performance liquid chromatography (HPLC) to examine changes in the levels of each hemoglobin species. Samples from ten patients treated with HU were analyzed approximately twice each week. Within the first 50 days of HU treatment, HbF increased on average from 3.3% too 7%. The patients had an average initial ratio G-gamma/A-gamma of 0.72 which did not change with treatment. At baseline, approximately 15-20% of the fetal hemoglobin exists in the acetylated form (F1) as determined by CE- HPLC, independent of subtype. With HU treatment, the total amount of F0 and F1 increased roughly proportional to the increase in F-reticulocyte numbers. Although the total HbA2 remained constant during treatment, there was an increase in a fraction which migrated with HbA2. In patients treated with combination HU and erythropoietin, we found a further increase in both F1 and this modified HbS fraction to levels approaching 5% and 10%, respectively.